1. Field of the Invention
The present invention relates to temperature stabilized resonant microwave cavities which do not require hermetic sealing and are easy to be frequency adjusted.
2. Statement of the Prior Art
It is known that at present oscillators and filters implement many types of microwave cavities with a metal wall and filled with gas, the most important ones of which are:
(1) TEM mode coaxial cavity; PA1 (2) TE.sub.10 mode waveguide cavity; PA1 (3) TE.sub.11 mode circular waveguide cavity; PA1 (4) TE.sub.01 mode circular waveguide cavity. PA1 (1) Cavity metal temperature expansion; PA1 (2) Dielectric constant of the gas filling the cavity; PA1 (3) Load impedance at the ports coupling the cavity to external circuits. PA1 TEM mode coaxial cavity with 1=.lambda./4 and 1=.lambda./2; PA1 TE.sub.101 mode rectangular waveguide cavity; PA1 TE.sub.010, TE.sub.111 and TE.sub.011 modes circular waveguide cavities. PA1 more cost saving due to a drastic simplification of the construction phases in that "difficult" alloys such as Invar and Superinvar are not used, which means saving of purchase and working costs.
It is also known that the biggest problem to be solved is the cavity resonance frequency stabilization upon a variation of environmental conditions (temperature and humidity) whenever a high frequency stability in the 1 ppm/.degree.C. is to be attained.
In fact, there generally are three fundamental factors affecting the resonance frequency of a cavity, ie:
As far as item (3) is concerned, the load effect becomes negligible by adequately reducing the coupling amount towards the load and where necessary, by introducing an isolator between cavity and load.
As to item (1) we point out that for manufacturing a cavity, a metal with a low expansion coefficient vs temperature is being used, i.e. Invar and Super Invar with an expansion coefficient less than/equal to 1.5 ppm/.degree.C. and less than/equal to 0.7 ppm/.degree.C., respectively.
In addition a particular heat treatment for stabilization of these materials is envisaged before and after their being worked. In this way, also the end product maintains the expansion coefficient values specified.
Finally concerning item (2) it is necessary to hermetically seal the cavity (i.e. it must be moist- and gasproof) before filling it with a dry inert gas (e.g. nitrogen) thus cancelling the difference in pressure with respect to the external environment. This solution is particularly hazardous as all soldering of the several parts constituting the cavity as well as the coupling irises and tuning adjustments must be sealed. In this view, Applicants have described in Italian patent application No. 26491 A/79 cavities not requiring gas filling in that the cavity metal wall is fitted with a quartz cylinder. In this patent application No. 26490 A/79 a description was given of cavities having an internal part with minor thickness made of precious alloy (Invar), whereas its external part is thicker and is made of a less precious alloy.